1. Field of the Invention
The present invention relates to a magnetic head of a magnetic tape drive.
2. Description of Relevant Art
In recent years, recording capacity of a magnetic recording medium has been developed, and a magnetic recording medium, which is used for saving data of computer and whose recording capacity is more than 100 G bytes, has been brought to the market.
In case of a magnetic tape, for example, the recording capacity thereof is increased by providing several hundreds of data tracks along a width direction of the magnetic tape. Thus, the distance between adjoining data tracks and the width of the data track become quite narrow as the recording capacity becomes large.
Therefore, the method disclosed in Japanese unexamined patent publication H08-30942, which is adopted in order to trace the data track accurately by a recording/writing element of a magnetic head, has been discovered.
In this Japanese patent publication, the magnetic head positioning servo control of the magnetic head, that is a positioning control in a width direction of the magnetic tape, is performed while reading a servo signal, which is preliminarily written on the magnetic tape, by a magnetic head.
The servo signal of these kinds is written on a servo band of the non-magnetized magnetic tape, by applying a recording current so that the direction of the magnetization is aligned in one direction.
That is, as shown in FIG. 6B, in case of conventional servo signal SS1, the servo signal is recorded on the non-magnetized magnetic tape, by applying a recording current PC1, which is composed of a zero current and a plus pulse current and which is shown in FIG. 6A, on the non-magnetized servo band SB1. This is because it is required to prevent the occurrence of the saturation of the magnetization of a servo read element (not shown).
According to this recording current PC1, the servo band of the magnetic tape MT1 is not magnetized while the recording current PC1 is a zero current, but the servo band is magnetized when a plus pulse current is supplied as the recording current PC1. In this occasion, the region where the plus pulse current is applied is magnetized in one direction and this region serves as the servo pattern SP1. Thereby, the servo signal SS1 composed of a group of servo patterns SP1 is written on the servo band SB1.
Here, a head gap of a magnetic head (not shown), which is used for writing the servo signal SS1, has a shape of non-parallel symmetrical patterns, and each pattern thereof has a predetermined intersection angle with respect to a traveling direction of the magnetic tape MT1. Therefore, as shown in FIG. 6A and FIG. 6B, the servo pattern SP1a being composed of non-parallel symmetrical patterns is formed by the magnetization by the plus pulse current PPa, and also the servo pattern SP1b is formed by the magnetization by the plus pulse current PPb.
In the magnetic tape drive, using a servo read element, the change of the magnetic field on the servo signal SS1 is detected. This servo read element detects the change of the magnetic field by measuring the change in electric resistance, and outputs the measured value as a readout signal in the form of the differential waveform (see FIG. 6C and FIG. 6D).
Here, since the readout signal is indicated as a value of voltage, the peak voltage of the readout signal becomes large as the change of the electric resistance measured by the servo read elements becomes large. Thus, the S/N (signal/noise) ratio of the readout signal is improved as the change of the electric resistance becomes large.
Therefore, as shown in FIG. 6C, the peak voltage of the readout signal RSL of the servo signal SS1 becomes large, when the variation of the magnetic field of the servo signal SS1 itself is large or the area to be traced by the servo read elements is wide because of the large width of the servo read element.
The recording capacity of the magnetic tape per volume may be increased to dozens of terabytes order in the near future. In this case, the number of the data tracks is increased in order to increase the recording capacity and thus the distance between adjoining data tracks and the width of the data track become narrow. Additionally, the thickness of the magnetic tape may be decreased together with the increasing of the recording capacity.
Thereby, the magnetic charge that can be detected while performing the readout of the servo signal SS1 is decreased, and the variation of the magnetic charge on the servo signal SS1 that are detected by the servo read element becomes small.
Thus, as shown in FIG. 6D, the peak voltage of the readout signal RSS of the servo signal SS1 becomes small, and the S/N ratio of the readout signal RSS becomes worse. These defects disturb the accurate readout of the servo signal SS1 and the accurate positional control of the magnetic head of the magnetic tape drive.
As an example of the technique for overcoming these defects, the technique disclosed in Japanese patent application No.2003-110396 (not yet published) can be cited.
In this patent application, by using an erase element (not shown) provided on the magnetic head of the servo writer, the servo signal SS2 being comprised of a plurality of servo patterns is written on a part of the servo band SB2, a region of which is being magnetized in one direction with respect to the longitudinal direction of the magnetic tape MT2.
In this case, a direction of the magnetization of each servo pattern of the servo signal SS2 is in the opposite direction with respect to that of the servo band SB2. That is, in case of the magnetic tape MT2 shown in FIG. 7A, the direction of the magnetization of the servo signal SS2 (servo pattern) is a left direction, and that of the servo band SB2 is a right direction. Here, referring to FIG. 7A, the direction of the magnetization is shown by a small arrow.
In this magnetic tape MT2 of shown in FIG. 7A, the direction of the magnetization is reversed at the boundary between the servo pattern SP2 and the servo band SB2. That is, the direction of the magnetization is reversed at the boundary between the servo band SB2 whose direction of the magnetization is a right direction in FIG. 7A and the servo pattern SP2 whose direction of the magnetization is a left direction in FIG. 7A.
These arrangements of the servo pattern SP2 enlarge the variation of the signal (readout signal) (see FIG. 7B) obtained by the readout of the servo signal SS2 that is performed by the servo read element S2 provided on the magnetic head H2 of the magnetic tape drive. Thus, the S/N ratio of the readout signal of the servo signal SS2 can be improved.
In other words, since the variation of readout signal at the boundary between the servo band SB2 and the servo pattern SP2 becomes large, the S/N ratio is improved.
In this technique, however, the magnetic head positioning servo control of the servo writer cannot be performed, when performing the magnetization of the servo band SB2 using the erase element in order to align in one direction the direction of the magnetization of servo band SB2. Hereinafter, aligning in one direction the direction of the magnetization is defined as “direct-current erase”.
This makes it difficult to perform the direct-current erase only on the servo band SB2. Thus, the data band DB2, onto which data signal is being recorded, is subjected to the direct-current erase in addition to the direct-current erase of the servo band SB2 when performing the direct-current erase on the servo band SB2.
Under this condition, if the recording of the data signal is performed using a recording element W2 provided on the magnetic head H2 of the magnetic tape drive and the reproducing of this data signal is performed by a reproducing element R2 provided on the magnetic head H2, the reproduced data signal may include a noise.
Here, an unevenly-coated magnetic particle layer of the magnetic tape MT2, that is, thickness variation of the magnetic particle layer is one of factors causing a noise.
FIG. 8 is a cross-sectional view of the magnetic tape MT2, in which a magnetic particle layer 21 is provided on a base 20.
Referring to FIG. 8, when performing the recording of data signal after the direct-current erase, if the thickness variation exists on the magnetic particle layer 21, data signal is only recorded on a surface layer 21a of the magnetic particle layer 21. That is, in this case, data signal is only recorded on the surface layer 21a at constant depth and is not recorded on a under layer 21b. Thus, the alignment of the direction of the magnetization caused by the direct-current erase is still remaining in the under layer 21b. 
As can be seen from FIG. 8, since the boundary face between the surface layer 21a and the under layer 21b is undulating due to the thickness variation of the magnetic particle, the magnetization of the under layer 21b gives an unfavorable influence on the surface layer 21a as a leakage flux. That is, this leakage flux causes a noise when performing the reproducing of data signal.
This noise especially may be large, if the thickness of the magnetic tape MT becomes thin due to the development of the high density recording technique. This is because the thickness variation of the magnetic particle layer will occur with ease as the thickness of the magnetic tape becomes thin.
As an example of measures for minimizing the occurrence of a noise, the magnetic tape drive disclosed in Japanese unexamined patent publication No. H07-182602 can be cited.
In this magnetic tape drive, an AC erase head (alternating current erase head) is provided at upstream, with respect to the travel direction of the magnetic tape, of the magnetic head, which is used for recording and reproducing of data signal.
According to this magnetic tape drive, the magnetic particles in the magnetic particle layer of the data band DB2 are, prior to the recording of data signal, aligned in a random direction by the AC erase head, even if the direct-current erase is performed on the data band DB2.
That is, the direction of the magnetization of the data band DB2 can be aligned in a random direction prior to the recording of data signal, even if the direct-current erase is performed on the data band DB2. Hereinafter, aligning in a random direction the direction of the magnetization is defined as “alternating-current erase”.
Thereby, since the magnetic particle layer is not being magnetized when performing the recording of data signal after the alternating-current erase (AC erase) of the data band DB2, only the surface layer of the magnetic tape is magnetized. Thus, only the data signal being recorded is reproduced when performing a reproducing, the occurrence of the noise at the time of reproducing can be minimized even if the thickness variation exist on the magnetic particle layer 21.
In these conventional magnetic tape drives, since the AC erase is performed on every data track of the data band DB2, the data track whose data is indispensable is also subjected to the AC erase. Additionally, since the servo signal SS2 being recorded on the servo band SB2 is also subjected to the AC erase, the magnetic head positioning servo control of the magnetic head H2 cannot be achieved.
Therefore, the magnetic head that can minimize the occurrence of a noise at the time of the reproducing of data signal has been required. Additionally, the magnetic tape drive adopting this magnetic head that can minimize the occurrence of a noise at the time of the reproducing of data signal, which is recorded on the data track after performing the AC erase only on the data track onto which data signal is recorded, has been required.